[unisog] Cisco Security Advisory: Cisco IOS DHCP Blocked Interface Denial-of-Service (fwd)

Lucy E. Lynch llynch at darkwing.uoregon.edu
Wed Nov 10 19:49:33 GMT 2004


Lucy E. Lynch 				Academic User Services
Computing Center			University of Oregon
llynch  @darkwing.uoregon.edu		(541) 346-1774/Cell: 912-7998

---------- Forwarded message ----------
Date: Wed, 10 Nov 2004 09:00:00 -0800
From: Cisco Systems Product Security Incident Response Team
    <psirt at cisco.com>
To: nanog at merit.edu
Cc: psirt at cisco.com
Subject: Cisco Security Advisory: Cisco IOS DHCP Blocked Interface

Hash: SHA1

Cisco Security Advisory: Cisco IOS DHCP Blocked Interface Denial-of-Service

Revision 1.0

For Public Release 2004 November 10 1700 UTC (GMT)

- -------------------------------------------------------------------------------


    Affected Products
    Software Versions and Fixes
    Obtaining Fixed Software
    Exploitation and Public Announcements
    Status of This Notice: FINAL
    Revision History
    Cisco Security Procedures

- -------------------------------------------------------------------------------


Cisco IOS devices running branches of Cisco IOS version 12.2S that have
Dynamic Host Configuration Protocol (DHCP) server or relay agent enabled, even
if not configured, are vulnerable to a denial of service where the input queue
becomes blocked when receiving specifically crafted DHCP packets. Cisco is
providing free fixed software to address this issue. There are also workarounds
to mitigate this vulnerability. This issue was introduced by the fix included
in CSCdx46180 and is being tracked by Cisco Bug ID CSCee50294 ( registered
customers only) .

This advisory is available at

Affected Products

This vulnerability was introduced by the fix for CSCdx46180, and was integrated
in Cisco IOS 12.2(14)SZ and 12.2(18)S.

The following Cisco products running Cisco IOS version 12.2(14)SZ, or a variant
of Cisco IOS 12.2(18)S (as listed in the following section) and higher are
affected by this vulnerability.

  * Cisco 7200, 7300, 7500 platforms
  * Cisco 2650, 2651, 2650XM, 2651XM Multiservice platform
  * Cisco ONS15530, ONS15540
  * Cisco Catalyst 4000, Sup2plus, Sup3, Sup4 and Sup5 modules
  * Cisco Catalyst 4500, Sup2Plus TS
  * Cisco Catalyst 4948, 2970, 3560, and 3750
  * Cisco Catalyst 6000, Sup2/MSFC2 and Sup720/MSFC3
  * Cisco 7600 Sup2/MSFC2 and Sup720/MSFC3

Vulnerable Products
- -------------------

This issue affects only Cisco devices running affected Cisco IOS versions 12.2
(18)EW, 12.2(18)EWA, 12.2(14)SZ, 12.2(18)S, 12.2(18)SE, 12.2(18)SV, 12.2(18)SW
and higher that do not have the configuration command no service dhcp. It is
not necessary for DHCP server or relay agent to be configured in order for this
vulnerability to be present and exploited; "service dhcp" is enabled by default
in IOS and is the only setting necessary (in addition to interface addresses)
for exploitation of this vulnerability. This includes routers as well as
switches and line cards which run Cisco IOS software. Cisco devices which do
not run Cisco IOS software are not affected. Cisco devices running affected
Cisco IOS software with the command no service dhcp enabled are not affected.

To determine the software running on a Cisco product, log in to the device and
issue the show version command to display the system banner. Cisco IOS software
will identify itself as "Internetwork Operating System Software" or simply
"IOS." On the next line of output, the image name will be displayed between
parentheses, followed by "Version" and the IOS release name. Other Cisco
devices will not have the show version command or will give different output.

The following example identifies a Cisco product running IOS release 12.0(3)
with an installed image name of C2500-IS-L:

    Cisco Internetwork Operating System Software IOS (TM)
    2500 Software (C2500-IS-L), Version 12.0(3), RELEASE SOFTWARE

The release train label is "12.0."

The next example shows a product running Cisco IOS release 12.0(2a)T1 with an
image name of C2600-JS-MZ:

    Cisco Internetwork Operating System Software IOS (tm)
    C2600 Software (C2600-JS-MZ), Version 12.0(2a)T1, RELEASE SOFTWARE (fc1)

Additional information about Cisco IOS release naming can be found at

Products Confirmed Not Vulnerable
- ---------------------------------

Cisco devices running affected Cisco IOS software with the command no service
dhcp enabled are not affected.

Cisco products that run any versions of IOS not listed in the Software Versions
and Fixes table below, are not affected.

Cisco products that do not run Cisco IOS software and are not affected by this
vulnerability include, but are not limited to:

  * 700 series dialup routers (750, 760, and 770 series) are not affected.
  * WAN switching products such as the IGX, BPX and MGX lines are not affected.
  * No host-based software is affected.
  * The Cisco PIX Firewall is not affected
  * The Cisco LocalDirector is not affected.
  * The Cisco Content Engine and ACNS is not affected.
  * The Catalyst 2901/2902, 2948G, 2980G, 4000, 5000, and 6000 switches running
  * Cisco Network Registrar is not affected.
  * Cisco VPN 3000 series is not affected
  * Cisco IOS-XR platform is not affected.
  * Cisco MDS 9000 family is not affected.


DHCP services allow devices to request and receive basic host configuration
information from the DHCP server via the network. Cisco routers can be
configured to both provide dynamic host configuration information (termed DHCP
server function), and forward DHCP and BootP packets across separate broadcast
domains (termed DHCP relay agent function). Cisco routers are configured to
process and accept DHCP packets by default, therefore the command "service
does not appear in the running configuration display, and only the command for
the disabled feature, no service dhcp, will appear in the running configuration
display when the feature is disabled. The vulnerability is present, regardless
if the DHCP server or relay agent configurations are present on an affected
product. The only required configuration for this vulnerability in affected
versions is the lack of the no service dhcp command. Certain crafted DHCP
packets may be undeliverable, but will remain in the queue instead of being
dropped. If a number of packets are sent that equal the size of the input
queue, no more traffic will be accepted on that interface.

On a blocked Ethernet interface, Address Resolution Protocol (ARP) times out
after a default time of four hours, and no inbound or outbound traffic can be
processed, including both IP and non-IP traffic such as IPX. The device must be
rebooted to clear the input queue on the interface, and will not reload without
user intervention. The attack may be repeated on all interfaces, causing the
router to be remotely inaccessible, excluding the console port where DHCP
packets are not processed by default and which can be used for out-of-band
management and configured for remote access. Workarounds are available, and are
documented in the Workarounds section below. Other types of interfaces,
including but not limited to ATM, Serial and POS interfaces, are affected, but
ARP is not a factor.

To identify a blocked input interface, use the show interfaces command and look
for the Input Queue line. The size of the input queue may keep increasing. If
the current size (in this case, 76) is larger than the maximum size (75), the
input queue is blocked.

    Router#show interface ethernet 0/0
    Ethernet0/0 is up, line protocol is up
      Hardware is AmdP2, address is 0050.500e.f1e0 (bia 0050.500e.f1e0)
      Internet address is
      MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, rely 255/255, load 1/255
      Encapsulation ARPA, loopback not set, keepalive set (10 sec)
      ARP type: ARPA, ARP Timeout 04:00:00
      Last input 00:00:41, output 00:00:07, output hang never
      Last clearing of "show interface" counters 00:07:18
      Input queue: 76/75/1091/0 (size/max/drops/flushes); Total output drops: 0

    !--- The 76/75 shows that this is blocked


A device receiving these specifically crafted DHCP packets will force the
inbound interface to stop processing traffic. The device may stop processing
packets destined to the router, including routing protocol packets and ARP
packets. No alarms will be triggered, nor will the router reload to correct
itself.  This vulnerability may be exercised repeatedly resulting in loss of
availability until a workaround has been applied or the device has been
upgraded to a fixed version of code.

The effects of this vulnerability can be monitored by the same methods outlined
in the white paper entitled Uses of Network Management for Monitoring the "IP
Packet Blocks Input Queue" PSIRT Advisory which details methods to identify
impacted devices via SNMP, RMON, and Network Management products.

Software Versions and Fixes

Each row of the Cisco IOS software table below describes a release train and
the platforms or products for which it is intended. If a given release train is
vulnerable, then the earliest possible releases that contain the fix (the
"First Fixed Release") and the anticipated date of availability for each are
listed in the "Rebuild" and "Maintenance" columns. A device running a release
in the given train that is earlier than the release in a specific column (less
than the First Fixed Release) is known to be vulnerable. The release should be
upgraded at least to the indicated release or a later version (greater than or
equal to the First Fixed Release label). When selecting a release, keep in mind
the following definitions:

  * Maintenance: Most heavily tested, stable, and highly recommended release of
    a release train in any given row of the table.
  * Rebuild: Constructed from the previous maintenance or major release in the
    same train, it contains the fix for a specific defect. Although it receives
    less testing, it contains only the minimal changes necessary to repair the

In all cases, customers should exercise caution to confirm that the devices to
be upgraded contain sufficient memory and that current hardware and software
configurations will continue to be supported properly by the new software
release. If the information is not clear, contact the Cisco TAC for assistance
as shown in the Obtaining Fixed Software section below.

More information on Cisco IOS software release names and abbreviations is
available at http://www.cisco.com/warp/public/620/1.html.

The fixes will be available at the Software Center located at

For software installation and upgrade procedures, see

For a current view of all posted and repaired images for Cisco IOS, please
check the listing available to registered Cisco.com users at:

| Major Release  |  Availability of Repaired  |
|                |          Releases          |
| Affected       |              |             |
| 12.2-Based     | Rebuild      | Maintenance |
| Release        |              |             |
| 12.2(18)EW     | 12.2(18)EW2  |             |
| 12.2(18)EWA    |              | 12.2(20)EWA |
|                | 12.2(18)S6   |             |
|                |--------------+-------------|
|                | 12.2(20)S4   |             |
|12.2(18)S       |--------------+-------------|
|                | 12.2(22)S2   |             |
|                |--------------+-------------|
|                |              | 12.2(25)S   |
| 12.2(18)SE     | 12.2(20)SE3  |             |
| 12.2(18)SV     |              | 12.2(24)SV  |
| 12.2(18)SW     |              | 12.2(25)SW  |
| 12.2(18)SXD    | Not impacted               |
| 12.2(14)SZ     | Migrates to  |             |
|                | 12.2(20)S4   |             |

Obtaining Fixed Software

Customers with Service Contracts
- --------------------------------

Customers with contracts should obtain upgraded software through their regular
update channels. For most customers, this means that upgrades should be
obtained through the Software Center on Cisco's worldwide website at http://

Customers using Third-party Support Organizations
- -------------------------------------------------

Customers whose Cisco products are provided or maintained through prior or
existing agreement with third-party support organizations such as Cisco
Partners, authorized resellers, or service providers should contact that
support organization for assistance with the upgrade, which should be free of

Customers without Service Contracts
- -----------------------------------

Customers who purchase direct from Cisco but who do not hold a Cisco service
contract and customers who purchase through third-party vendors but are
unsuccessful at obtaining fixed software through their point of sale should get
their upgrades by contacting the Cisco Technical Assistance Center (TAC). TAC
contacts are as follows.

  * +1 800 553 2447 (toll free from within North America)
  * +1 408 526 7209 (toll call from anywhere in the world)
  * e-mail: tac at cisco.com

Please have your product serial number available and give the URL of this
notice as evidence of your entitlement to a free upgrade. Free upgrades for
non-contract customers must be requested through the TAC.

Please do not contact either "psirt at cisco.com" or "security-alert at cisco.com"
for software upgrades.

See http://www.cisco.com/warp/public/687/Directory/DirTAC.shtml for additional
TAC contact information, including special localized telephone numbers and
instructions and e-mail addresses for use in various languages.

Customers may only install and expect support for the feature sets they have
purchased. By installing, downloading, accessing or otherwise using such
software upgrades, customers agree to be bound by the terms of Cisco's software
license terms found at http://www.cisco.com/public/sw-license-agreement.html,
or as otherwise set forth at Cisco.com Downloads at


There are four possible workarounds for this vulnerability:

  * Disabling the dhcp service
  * Control Plane Policing
  * Two versions of Access Control Lists

Disabling the DHCP Service
- --------------------------

This vulnerability can be mitigated by utilizing the command:

    no service dhcp

However, this workaround will disable all DHCP processing on the device,
including the DHCP helper functionality that may be necessary in some network

Control Plane Policing Feature
- ------------------------------

The Control Plane Policy feature may be used to mitigate this vulnerability, as
in the following example:

    access-list 140 deny   udp host any eq bootps
    access-list 140 deny   udp any host eq bootps
    access-list 140 deny   udp any host eq bootps
    access-list 140 permit udp any any eq bootps

    class-map match-all bootps-class
      match access-group 140

    policy-map control-plane-policy
      class bootps-class

         police 8000 1500 1500 conform-action drop exceed-action drop

     service-policy input control-plane-policy

For this example is a legitimate DHCP server.

Additional information on the configuration and use of the CPP feature can be
found at this link:


This workaround is only applicable to Cisco IOS 12.2S, as this feature is only
available in Cisco IOS versions 12.2S and 12.3T. Cisco IOS 12.3T is not
impacted by this advisory.

Access Lists - Two Methods
- --------------------------

Access lists can be applied to block DHCP/BootP traffic destined to any router
interface addresses, as in the following example:

In this example, the IP address represents a legitimate DHCP
server, the addresses and represent router interface
addresses, and represents a loopback interface on the router.

In this example, any bootp/dhcp packets destined to the router interface
addresses are blocked.

    access-list 100 remark permit bootps from the DHCP server
    access-list 100 permit udp host any eq bootps
    access-list 100 remark deny bootps from any to router f1/0
    access-list 100 deny   udp any host eq bootps
    access-list 100 remark deny bootps from any to router f0/0
    access-list 100 deny   udp any host eq bootps
    access-list 100 remark deny bootps from any to router loopback1
    access-list 100 deny   udp any host eq bootps
    access-list 100 remark permit all other traffic
    access-list 100 permit ip any any

access-list 100 is applied to f0/0 and f1/0 physical interfaces.

    interface FastEthernet0/0
     ip address
     ip access-group 100 in
    interface FastEthernet1/0
     ip address
     ip access-group 100 in
     ip helper-address

An alternate configuration for the interface access-list workaround.

This example would also need to be applied to all physical interfaces, but deny
statements for all of the IP addresses configured on the router are not
necessary in this approach. In this example, the address
represents a legitimate DHCP server.

    access-list 100 permit udp host any eq bootps
    access-list 100 permit udp any host eq bootps
    access-list 100 permit udp any host eq bootps
    access-list 100 deny   udp any any eq bootps

    interface FastEthernet0/0
     ip address
     ip access-group 100 in
    interface FastEthernet1/0
     ip address
     ip access-group 100 in
     ip helper-address

Note: These workarounds will not prevent spoofed IP packets with the source IP
address set to that of the DHCP server

For more information on anti-spoofing refer to http://www.cisco.com/en/US/tech/
tk648/tk361/technologies_tech_note09186a0080120f 48.shtml#sec_ip and

The Unicast Reverse Path Forwarding (Unicast RPF) feature helps to mitigate
problems that are caused by spoofed IP source addresses.  For further details,
please refer to http://www.cisco.com/univercd/cc/td/doc/product/software/ios122

Exploitation and Public Announcements

The Cisco PSIRT is not aware of any public announcements or malicious use of
the vulnerability described in this advisory.  This vulnerability was reported
to Cisco by a customer.

Status of This Notice: FINAL



This advisory will be posted on Cisco's worldwide website at

In addition to worldwide web posting, a text version of this notice is
clear-signed with the Cisco PSIRT PGP key and is posted to the following e-mail
and Usenet news recipients.

  * cust-security-announce at cisco.com
  * first-teams at first.org (includes CERT/CC)
  * bugtraq at securityfocus.com
  * vulnwatch at vulnwatch.org
  * cisco at spot.colorado.edu
  * cisco-nsp at puck.nether.net
  * full-disclosure at lists.netsys.com
  * comp.dcom.sys.cisco at newsgate.cisco.com

Future updates of this advisory, if any, will be placed on Cisco's worldwide
website, but may or may not be actively announced on mailing lists or
newsgroups. Users concerned about this problem are encouraged to check the
above URL for any updates.

Revision History

| Revision |                  | Initial       |
| 1.0      | 2004-November-10 | public        |
|          |                  | release.      |

Cisco Security Procedures

Complete information on reporting security vulnerabilities in Cisco products,
obtaining assistance with security incidents, and registering to receive
security information from Cisco, is available on Cisco's worldwide website at
This includes instructions for press inquiries regarding Cisco security
notices. All Cisco security advisories are available at

- -------------------------------------------------------------------------------

All contents are Copyright © 1992-2004 Cisco Systems, Inc. All rights reserved.
Important Notices and Privacy Statement.

Version: PGP 7.0


More information about the unisog mailing list